Liquid nitrogen and liquid CO.sub.2 are conventionally utilized to cool the interiors of insulated containers as well as materials stored therein for the purpose of maintaining low temperatures of the materials and the interior of the insulated containers, especially when the containers are to be used in shipment of the materials therein over extended periods of time without benefit of mechanical or absorption type refrigeration systems. In addition, insulated containers are also conventionally provided with cold holdover plates which may be refrigerated by suitable mechanical or absorption type refrigeration equipment at a starting point and thereafter disconnected from the refrigeration equipment and utilized to maintain the interior of a container in which they are disposed as well as other contents of the container refrigerated over extended periods of time.
The utilization of liquefied nitrogen and carbon dioxide as well as cold holdover plates is presently enjoying increased popularity, inasmuch as transit type containers refrigerated by mechanical or absorption type refrigeration systems often experience malfunction of the attendant refrigeration units and the contents of the refrigerated container is subject to spoilage.
The utilization of liquefied nitrogen for refrigerating the interiors of shipping containers is usually employed when very low freezing temperatures are desired within the container and cold holdover plates are usually used for refrigerating containers when cooling or mild freezing temperatures within the container are required. The use of liquefied carbon dioxide for refrigerating shipping containers is generally limited to containers in which cooling or moderately low freezing temperatures are required and the liquefied carbon dioxide is, in most cases, utilized to form carbon dioxide snow which is approximately minus 69.degree. F., at the triple point pressure of 75 lbs. per square inch absolute, but which drops from minus 69.degree. F. to minus 109.degree. F. at atmospheric pressure. Further, CO.sub.2 snow is conventionally formed by discharging chilled liquid carbon dioxide from spray nozzles. In such instance, the liquid carbon dioxide is supplied to the spray nozzles at approximately 0.degree. F. with the result that one lb. of liquid carbon dioxide will yield approximately 0.40 to 0.45 lbs. of carbon dioxide snow. However, if the temperature and pressure of the liquid carbon dioxide can be maintained at the triple point wherein the pressure thereof is approximately 75 lbs. per square inch absolute and the temperature is approximately minus 69.degree. F., one pound of liquid carbon dioxide will yield approximately 0.46 to 0.51 lbs. of carbon dioxide snow.
There are three distinct thermal cycles which may be used in the manufacture of solid carbon dioxide (carbon dioxide snow). These distinct thermal cycles include the non-regenerative, the regenerative and the complete re-expansion cycles. However, each of these cycles requires considerable machinery and are thus not practiced to any great extent in the production of carbon dioxide snow to be used in insulated transport containers.
Accordingly, a need exists for an improved, simple and efficient method of producing a maximum amount of carbon dioxide snow from a given amount of liquid carbon dioxide.
Examples of carbon dioxide snow producing machines including some of the general structural and operational features of the instant invention are disclosed in U.S. Pat. Nos. 2,247,850, 2,483,064, 2,731,807, 3,561,226, 3,695,056 and 3,815,377.